The present invention relates to a magnetic disk apparatus of embedded servo system, wherein the same disk surface carries both servo information and user data, and particularly to a magnetic disk apparatus which is capable of efficiently recording and reproducing data in such applications that data is recorded and reproduced in a large unit as video data.
Magnetic disk apparatuses which utilize magnetic disks, particularly hard disk apparatuses (HDD) have recently been advancing at an amazing rate toward higher speed and larger storage capacity. As a result, magnetic disk apparatuses having storage capacities of 10 GB per unit have been commercialized and utilized for recording and reproducing AV data as well, in nonlinear video editing system or the like. When using AV data, HDD has been required to have, in addition to a larger storage capacity, improved random access capability and securing proper recording and reproducing speed to prevent edited video data from being interrupted.
The hard disk apparatus of the prior art mentioned above will be described below by using reference to the accompanying drawings.
FIG. 6 shows the recording system employed in the hard disk of the prior art as a recording medium. The hard disk shown in FIG. 6 is the recording medium wherein data is recorded by a system called embedded servo system. In FIG. 6, recording zones 41 are formed by dividing the disk surface in the radial direction into a plurality of annular regions so that a larger amount of data can be recorded in a zone having longer circumference. Each of the recording zones 41 comprises a plurality of tracks, and every track in one recording zone 41 has the same storage capacity for recorded data. Such recording system is the embedded servo recording system.
In FIG. 6, a servo region 42 holds information recorded therein to position a magnetic head (not shown in the drawing) on a track in the prior art. The hard disk is divided in the rotating direction into a plurality of recording segments 40 having substantially the same angles. One of the recording segments 40 is further divided into the servo region 42 and a data region 43. Number of the divisions is determined according to the tracking control performance of the magnetic head in locating tracks. One of design parameters of tracking control system for locating the tracks is gain-crossover frequency which is set in a range from about 400 Hz to 600 Hz in many cases of hard disk apparatus.
Off-track information is detected at a sampling frequency determined by the rotating speed of the disk and the number of divisions of the recording segment 40. The sampling frequency is set to about ten times the gain-crossover frequency. For example, when the gain-crossover frequency is 500 Hz and the disk rotating speed is 5400 rpm (i.e. 90 Hz), the number of dividing of recording segment 40 is about 56 (.apprxeq.500/90) sections. On the condition that the recording segment 40 is divided into 56 sections, the sampling frequency is set at 90.times.56=5040(Hz), which is about ten times the gain-crossover frequency.
FIG. 7 is a drawing to explain the servo region 42 in the hard disk apparatus (HDD) of the prior art. The servo region 42 includes a synchronization region 45 located at the head thereof, for the purpose of pretreatment necessary to reproduce data. The synchronization region 45 holds recorded pattern for detecting such information as AGC (auto gain control), PLL (phase lock loop) and the servo region.
An identifier region 46 holds a track identifying code, which is required for carrying out seek control, recorded therein. Recently the number of hard disk apparatuses (HDD) provided with MR head (magnetoresistive head) has been increasing. Position of an MR head must be a little different between times when recording and reproducing, depending on the track. For this reason, such a unique identifier code has been recorded in the data region 43 that indicates the position in each sector which is the unit of recording and reproduction. In the case of the MR head, however, because the magnetic head must be shifted a little during recording, the identifier code becomes off-track, thus making it impossible to reproduce. Therefore, increasing HDDs provided with the MR head employ non-ID system wherein a code which is unique over the disk surface is recorded in the identifier region 46 and is used as the identifier region.
A burst region 47 in the servo region 42 holds a pattern recorded therein which is capable of detecting the excursion of the magnetic head from position with a high sensitivity.
In FIG. 6 mentioned previously, the arc-shaped data region 43 is usually divided into a plurality of sectors 44 so that user data can be recorded and reproduced in the unit of 512B (byte).
FIG. 8 is a drawing to explain the layout of the sectors 44 in the data region 43 of the prior art. In FIG. 8, (A) shows the layout of the sectors in an outer zone in the data region 43 and (B) shows the layout of the sectors in an inner zone in the data region 43. As shown in FIG. 8, the outer zone is longer than the inner zone and therefore holds larger amount of data recorded therein. In the case of this example, data of three sectors plus is recorded in the data region 43 in the outer zone, while data of two sectors plus is recorded in the inner region. Total recording capacity of the data region 43 is determined by the recording/reproducing characteristics of the magnetic disk as the recording medium and of the magnetic head, and is set so that the recording efficiency is maximized.
It is preferable to set the recording density of the data region 43 so that an integer number of sectors 44 are included in the data region 43, in that no sector is divided into fractions. However, as shown in FIG. 8, recording efficiency is improved by employing such a layout as some sectors 44 lie over adjacent servo regions 42. While each zone includes a plurality of tracks, sectors 44 are arranged in every zone similarly by taking the specified servo region 42 as the start point.
Now a procedure of making access to the sector 44 in the conventional apparatus will be described below.
In the hard disk apparatus of the prior art, information indicating how many sectors per track are formed in each zone can be recognized by a CPU incorporated in the apparatus by using such means as recording in ROM. Because the identifier 46 of the servo region 42 shown in FIG. 7 includes a code which is unique over the disk surface, the hard disk apparatus is capable of making access to any desired one of the servo regions 42. The hard disk apparatus also has the layout information of the sectors 44 in each of the data regions 43 in the form of a table stored in the ROM. Thus the hard disk apparatus is capable of making access to any desired one of the sectors 44 by using the information described above. Because the layout information of the sectors 44 is the same for different data regions 43 in a zone, it suffices to provide a table for each zone.
FIG. 9 shows the detailed structure of the sectors 44 formed in the data region 43 of the prior art.
The first servo region 42 is followed by a pattern which is required by an AGC circuit and is recorded in an AGC region 48. The AGC circuit is a circuit that maintains the amplitude of data reproduction signal constant after being amplified. Because an off-track error is detected by using the amplitude of the reproduction signal of the burst region 47 (FIG. 7) included in the servo region 42, the AGC circuit must be stopped at least in the burst region 47. Therefore the AGC region 48 is necessary for the restoration time of the AGC circuit which is required to reproduce data.
The AGC region 48 is followed by a synchronization region 49. The synchronization region 49 is a region required for clock synchronization of recorded data. In the case of a hard disk apparatus which employs the PRML (partial response maximum likelihood) recording system which has recently been put in commercial use, reproduction waveform is equalized by using a filter for circuit thereby to improve the reproduced data. In some hard disk apparatuses, the equalization parameter is determined by learning from the reproduction waveform. In such a hard disk apparatus, a pattern required for the learning is recorded in the synchronization region 49.
Next to the synchronization region 49, a DAM (data address mark) region 50 is provided. The DAM region 50 indicates the start point of user data. The data is recorded in next data recording region 51 with bit serial format and the reproduced data is processed in the unit of byte which usually consists of eight bits. Therefore, the DAM region 50 is important also for converting bit data into byte.
The data recording region 51 in the hard disk apparatus of the prior art has such a configuration as data is recorded in the fixed unit of 512B. Recorded in an ECC (error correcting code) region 52 is a code capable of detecting and correcting errors by using parity data recorded in the ECC region 52, even when the error is in the data recording region 51. A GAP region 53 is a region provided for preventing the head of the second sector indicated with numeral 83 from being destroyed by overwriting in the head of the second sector 83 due to variation in the rotation of a motor or the like when recording in the first sector indicated with numeral 82. The first sector 82 comprises from the synchronization region 49 through the GAP region 53 as shown in the drawing, and the next second sector 83 has the same configuration. The third sector 84 has a configuration which is different from those of the first sector 82 and the second sector 83 because the data region 51 lies over adjacent servo regions 42. In the data region 51 of the third sector 84, data of 512B is divided into predetermined two fractions, with the data in the first fraction being written before the servo region 42 as shown in the drawing. At this time, in order to prevent the servo region 42 from being destroyed by recording of data, the GAP region 53, having a function equivalent to that of the GAP region 53 provided at the end of the second sector 83, is provided before the servo region. The other divided fraction is placed after the servo region 42, although data is recorded first in the AGC region 48, the synchronization region 49 and the DAM region 50, and thereafter the rest of the data is recorded, because the regions for the AGC process, clock synchronization and byte synchronization are necessary.
FIG. 10 is a drawing to explain a method of generating error correction code in the hard disk apparatus of the prior art. In FIG. 10, (A) shows the method of generating error correction code by using the conventional reed-solomon code, and (B) shows the layout of recorded data in the hard disk.
As shown in (A) of FIG. 10, recorded data of 512 bytes from D.sub.0 through D.sub.511 are arranged successively in the unit of four bytes in the interleaving direction and are divided into four frames. Then for the user data of 128 bytes in the frame direction of each frame, parity codes P.sub.0 P.sub.1, . . . , P.sub.15 for error correction of four bytes are generated and arranged as shown in (A) of FIG. 10. In case the 4-byte error correction codes are generated as described above, errors can be corrected even when the 128 bytes in the frame direction and the 4 bytes of error correction code, 132 bytes in total, include an error of 2 bytes.
(B) of FIG. 10 shows the layout of data recorded on the disk surface. The data layout recorded on the disk surface is such that D.sub.0, D.sub.1, . . . , D.sub.511 are recorded in the data region 51 followed by the error correction codes recorded in the ECC region 52 in the order of P.sub.0, P.sub.1, . . . , P.sub.15.
Recently, as the storage capacities of hard disks increase, hard disks are increasingly used to store video data of nonlinear video editing system, video camera or the like. In such uses, for example, in case of motion picture data of JPEG format, it is desirable to control the recording and reproduction in the unit of frame. However, since the unit of recording and reproducing data in the conventional hard disk is fixed to 512B, it has been necessary to control the recording length for one frame in the unit of 512B in the application system. For example, when video data of one frame is divided into 640 segments in the horizontal direction and into 480 segments in the vertical direction while each segment is represented by one byte for each of red (R), green (G) and blue (B) color information, 640.times.480.times.3=921600 bytes of data is required for every frame. When recording this amount of data in a disk, it is common to reduce the amount of data by using a compression technique such as motion picture JPEG. When data is compressed to 1/10, for example, the amount of video data becomes 92160 bytes per frame. Thus data recorded in the conventional hard disk is controlled in the unit of 92160/512=180 sectors.
Recently, as the recording density of the hard disk increases, pit cells of recorded data has been becoming smaller. As a result, effects of unevenness and missing magnetic material on the magnetic disk are becoming conspicuous. Consequently, data errors have been increasing and S/N ratio has been decreasing due to smaller bit cells of recorded data, resulting in lower data quality. Also such a problem has been increasingly experienced as the speed of recording or reproducing data decreases due to the retry process, namely the operation of reading over erroneous data. In order to overcome such problems, error correction procedure is improved in many apparatuses. Efficiency of error correction can be improved by processing data in a larger unit. The hard disk apparatus of the prior art has a limitation in achieving efficient error correction process because the sector size is fixed to 512B. The magnetic disk apparatus of the present invention is capable of correcting errors efficiently even for a large sector size such as video data, thereby improving the reliability of data, and is further capable of easily processing sectors of variable length.